1. Field of the Invention
This invention relates to improvements in a mask used for forming a thin film having of a specified pattern, in dry type thin film forming methods such as sputtering, vapor deposition, ion plating and the like.
2. Prior Art
FIG. 3 is a sectional view illustrating a mask for use in vapor deposition, which is one example of a conventional thin film forming mask. A mask 1 is formed with openings 2 for forming a desired pattern. The mask 1 is generally made of a metal material such as stainless steel, which is excellent is corrosion resistance. In vapor deposition, as shown in FIG. 3, the mask 1 is placed on one surface of a workpiece 4 to be coated by vapor deposition and is fixed by bolts 5 and nuts 6 to the workpiece 4. The reason for this procedure is that in order to form a desired pattern exactly on the workpiece 4 by vapor deposition, it is necessary to secure the mask 1 positively to the entire surface of the workpiece 4 on which a thin film is to be formed.
The above prior art example relates to a mask 1 for use in vapor deposition. Similar thin film forming masks have been used as well in other dry type thin film forming methods such as sputtering, ion plating and the like.
The general practice is for the mask 1 to be heated together with the workpiece 4 at a temperature of 200.degree. to 500.degree. C. when a film is to be formed by dry type thin film forming methods such as sputtering, vapor deposition, ion plating and the like. Accordingly, as shown in section in FIG. 4, during the actual stage of thin film formation, a problem may occur in that this heating may flex the mask 1. Also, when the mask 1 is fastened to the workpiece 4 by bolts 5, an excessively strong fastening force exerted by the bolts may sometimes flex the mask 1. As a result, especially in the portion away from the bolts 5, the mask 1 may be considerably raised off the surface of the workpiece 4 on which a thin film is to be formed.
Although an increase in the number of bolts 5 improves adhesibility of the mask 1 to the work 4, on the other hand the increased number of bolts 5 reduces the effective area of vapor coating, resulting in reduction in productivity. Furthermore, attachment and removal of many bolts in each vapor deposition procedure reduces working efficiency, resulting in greater reduction in productivity.
On the other hand, in some cases the workpiece itself flexes. FIG. 5 is a plan view, and FIG. 6 is a sectional view taken along the line VI--VI in FIG. 5, illustrating an example of flexion of the workpiece itself. The structure shown in FIGS. 5 and 6 is a piezoelectric transducer in which a piezoelectric crystal thin film 8 made of zinc oxide (ZnO) is formed on an upper surface of a base material 7 such as Elinvar (trade name), and electrodes 9a and 9b are formed by a well-known thin film forming method. In constructing this piezoelectric transducer, during the time the ZnO thin film 8 is being formed on the upper surface of the base material 7 of Elinvar, the material 7 often flexes to project in the direction of the ZnO thin film 8 due to the grain growth of ZnO.
FIG. 7 is a schematic side view of an arrangement for forming the electrodes 9a and 9b, wherein a structure 10 including a ZnO thin film 8 formed on the upper surface of a base material 7, is placed on a support portion 10A, and the mask 1 is fixed to the upper surface of the support portion 10A. In this arrangement, it is difficult to form electrodes exactly in accordance with a desired pattern because the structure 10 itself, namely the workpiece to be treated, flexes as shown. Since in this case, unlike the case shown in FIGS. 3 and 4, the workpiece itself flexes and projects toward the mask 1, it becomes impossible to form electrodes in accordance with a desired pattern in the peripheral area of the workpiece, since the mask contacts primarily the central area of the workpiece.
The problems described above are not limited to a mask used in vapor deposition, but are likewise true of masks for use in other dry type thin film forming methods such as sputtering, ion plating and the like.